The term “polymeric composition with photovoltaic properties” means any composition comprising at least one polymeric material that is capable of directly converting light energy, in particular from solar radiation, into positive and negative electrical charges or bound electron-hole pairs, and of ensuring their dissociation and their transport in order to generate a photocurrent.
The principle of direct conversion of solar energy into electrical energy was discovered by Becquerel in 1839 and the first silicon-based solar cells saw the light of day in the mid-1950s. Currently, photovoltaic cells are essentially composed of inorganic semiconductor materials such as gallium arsenide or crystalline silicon. The conversion efficiencies of the best inorganic photovoltaic cells that are currently commercially available are typically of the order of 10% to 15%. Even though the maximum theoretical efficiency of a silicon monojunction cell, which is close to 30%, means that improvements in performance can be aspired to, the high cost associated with the fabrication of cells of that type remains an impediment to their large-scale market penetration.
In this regard, semiconductor polymers and organic molecules offer an interesting alternative because of their low production costs and of processing techniques that are inaccessible to inorganic materials. Organic and polymeric molecules are easy to handle and selecting them as a base material means that only one technology is required in order to engineer the cell (i.e. from the substrate to the protective capsule). Further, polymers are degradable, which ensures technology that is clean in the longterm.
Since the publication in 1986 by Tang (C. W. Tang: Two-layer organic photovoltaic solar cell. Appl. Phys. Lett., 48, 183, 1986; U.S. Pat. No. 4,164,431) which discloses a bilayer heterojunction with a power conversion efficiency of close to 1%, huge strides have been taken in work in the organic materials field.
Another advance occurred with the discovery of effective charge transfer between C60 and a semiconductor polymer, MEH-PPV, by Sariciftci et al in 1992 (N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science, 258, 1474, 1992), and then the production of the first MEH-PP V/C60 bilayer heterojunction cell in 1993 par Saricitftci et al. (Appl. Phys. Lett., 62, 585, 1993).
In 1995, the first bulk heterojunctions were produced from solutions containing the donor and acceptor (G. Yu, A. J. Heeger, J. Appl. Phys., 78, 4510, 1995; J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Holmes, Nature, 376, 498, 1995). In those cells, the interface between the p-type semiconductor and the n-type semiconductor is distributed throughout the volume of the active layer, which produces good charge separation from photo-generated excitons that have mobility that is very low in organic materials. This concept of interpenetrating networks between two compounds, respectively the electron donor and electron acceptor, has been widely applied, and a wide variety of chemical structures, from molecular species to polymers, has been explored.
Exciton dissociation is improved when photogeneration sites are bulk or heterojunction distributed. Such a configuration is obtained by organizing the donor/acceptor type materials in interpenetrated networks, which increases the surface area of the junction.
Apart from the fact that the mobilities of holes and of electrons in donor and acceptor materials must be as high as possible, the organization of the materials into interpenetrating networks facilitates charge conduction towards the electrodes. As a general rule, charge transport improves as molecular order increases.
The challenge, then, is to organize those donor and acceptor materials into interpenetrating networks in order to optimize the dissociation surface area of excitons and to encourage charge conduction without the appearance of recombination phenomena. Recombination phenomena limit conduction and collection of charge at the electrodes of the photovoltaic devices.
Currently, the most promising photovoltaic cells combine fullerenes and electron donor type polymers in interpenetrating networks. As an example, photovoltaic cells with an active layer formed by an association of a soluble conjugated electron donor type polymer, poly-(3-hexylthiophene) (P3HT) and a soluble electron acceptor type buckminsterfullerene derivative (PCBM) have conversion efficiencies that approach 5%. Their efficiency is not only due to the intrinsic properties of the materials, but also to the molecular order and the spatial organization of the active layer. P3HT organizes itself into a matrix of nanocrystalline structures that provide good hole conduction properties. The second material, PCBM, which is a C60 derivative, is integrated with the matrix and provides good electron conduction.
Recently, Yang et al have shown that the molecular order of active layers comprising a blend of P3HT/PCBM is dependent on the fabrication conditions (X. Yang, et al, Nanoletters, 5, 579, 2005). P3HT/PCBM active layers actually have a different architecture depending on whether the P3HT/PCBM blend has or has not undergone annealing after deposition onto a substrate. After annealing, the mixture becomes heterogeneous, with zones rich in PCBM and zones rich in polymer in which the polymer chains form fibrillar structures. In contrast, short fibrillar structures, which are not connected together, appear in the absence of annealing. Yang et al established the paradigm that an annealing step is essential to producing an organization that can produce photovoltaic cells presenting high efficiency. In fact, annealing can control the shape and organization of the chains of polymers into nanofibrils, thereby creating an interpenetrating network with the acceptor material.
The term “annealing” means a heat treatment comprising heating to, and holding at, an appropriate temperature followed by slow cooling, resulting in a structural constitution that is close to equilibrium.
However, some of the substrates used as donor/acceptor thin layer supports in photovoltaic cells cannot tolerate the temperatures employed during annealing.